What does `false` in `NonlinearProblem{false}` mean?

[dlfivefifty]

The README is a bit sparse... and the docstring doesn't help:

help?> NonlinearProblem
search: NonlinearProblem

  struct NonlinearProblem{uType, isinplace, P, F, K} <: SciMLBase.AbstractNonlinearProblem{uType, isinplace}

  ───────────────────────────────────────────────────────

  NonlinearProblem(f, u0)
  NonlinearProblem(f, u0, p; kwargs...)
  

  Define a steady state problem using an instance of
  AbstractNonlinearFunction.

  ───────────────────────────────────────────────────────

  NonlinearProblem(prob)
  

  Define a steady state problem from a standard ODE
  problem.

[ChrisRackauckas]

It's for whether the function f is in-place f(du,u,p) or out-of-place du = f(u,p). It's almost always determined automatically but we should probably document it anyways.